Take out the battery, flip the machine over and '''check the ribbon cable that runs from the trackpad to the keyboard''', to the connector. '''Known issues are:''' loose at connection point, or ribbon could be wrinkled, lose, crimped so that pressing keys moves the [http://www.takeitapart.net/archives/macbook-pro|connector].
Some people have had luck just '''pressing it flat''' against the keyboard,''(especially where it passes through the slot in the sidewall)'' other have '''shimmed the cable''' using a folded sticky note beween the cable and the battery.
Good Luck,
N.